1. Field of the Invention
The present invention relates to a transparent heat-shielding/heat-insulating member and a production method thereof.
2. Description of Related Art
From the viewpoint of preventing global warming and saving energy, blocking heat rays from sunlight (infrared rays) through the windows of buildings, shop windows, the windows of vehicles and the like is commonly performed to reduce the indoor temperature (Patent Document 1: JP 2014-170171 A). In addition, recently, from the viewpoint of saving energy, heat-shielding/heat-insulating members that have not only heat shielding properties that block heat rays that cause a temperature increase in summer, but also a heat-insulating function that suppresses the escape of heat from inside the room in winter and reduces the heating load have been proposed and are introduced into the market (Patent Document 2: JP 2014-141015 A, Patent Document 3: JP 2014-167617 A).
Patent Document 1 discloses a transparent heat-shielding film that has an anti-reflection function and in which a hardcoat layer, an infrared absorbing layer, a high refractive index layer and a low refractive index layer are sequentially stacked on a transparent substrate. The transparent heat-shielding film disclosed in Patent Document 1 is a transparent heat-shielding film of infrared absorbing type that absorbs infrared rays entering from the outside, and therefore does not have a heat-insulating function that reflects far-infrared rays having a wavelength of 5 to 25 μm emitted from a heater toward the inside of the room in winter.
Patent Document 2 discloses an infrared reflective stacked film in which a heat ray reflective layer and a hardcoat layer are stacked in order on a substrate, the heat ray reflective layer having a multilayer structure in which a thin metal film and a thin metal oxide film are alternately stacked. The stacked film disclosed in Patent Document 2 is an infrared reflective stacked film, and has a heat-insulating function that reflects infrared rays toward the inside of the room. However, when the thickness of the hardcoat layer is reduced in order to suppress the absorption of infrared rays and allow the heat-insulating function to work, particularly the thickness of the hardcoat layer is reduced to several hundred nanometers, which overlaps the wavelength range of visible light (380 to 780 nm), even the presence of a slight variation in the thickness of the hardcoat layer produces a noticeable glittering phenomenon in the appearance called “iridescent phenomenon” caused by a multi-reflective interference between interfacial reflection at the hardcoat layer and interfacial reflection at the heat ray reflective layer, and also increases a change in the reflection color due to a change in the optical path length when viewed from a different angle, which may present a problem of appearance when it is used by being attached to a window or the like.
Patent Document 3 discloses an infrared reflective film including an infrared reflective layer and a transparent protective layer in this order on a transparent film substrate, the infrared reflective layer including a first metal oxide layer, a metal layer and a second metal oxide layer in this order, and the transparent protective layer including an organic layer. The infrared reflective film disclosed in Patent Document 3 is of infrared reflective type, and has a heat-insulating function that reflects infrared rays toward the inside of the room. However, if the thickness of the transparent protective layer is changed to 150 nm or less, which is smaller than the wavelength range of visible light, in order to suppress the iridescent phenomenon that occurs in the appearance, physical properties such as scratch resistance tend to decrease, and scratches are likely to be generated on the film surface at the time of the application of the film or while the film is used for a long period of time, which may present problems caused by the scratches such as poor appearance and corrosion.
In an infrared reflective heat-shielding film including an infrared reflective layer composed of a stack of a thin metal film and a thin metal oxide film as disclosed in Patent Documents 2 and 3, the thin metal film usually has an infrared reflecting function and is formed of a low refractive index layer that has a high visible light transmittance, and the thin metal oxide film has a protection function that, while maintaining the infrared reflecting function of the thin metal film, controls the reflectance at a wavelength in the visible light range so as to increase the transmittance in the visible light range and that suppresses migration of metals in the thin metal film, and is usually formed from a material having a refractive index as high as 1.7 or more.
Accordingly, when a UV hardcoat layer made of an acrylic resin having a refractive index of, for example, around 1.5, which is usually used as a protective layer, is formed on the infrared reflective layer composed of a stack of a thin metal film and a thin metal oxide film, a multi-reflective interference occurs at each interface due to the difference in refractive index between each layer of the infrared reflective layer and the hardcoat layer and the thickness of each layer. As a result, the reflectance at each wavelength of visible light incident on the infrared reflective film varies significantly. That is, when a visible light reflection spectrum of the infrared reflective film is obtained, a reflectance curve having a so-called “ripple”, which is a shape with significant fluctuations of peaks and valleys, is observed.
Normally, a protective layer, such as a UV hardcoat layer, made of an acrylic resin is applied and formed by a wet coating method, and it is practically difficult to uniformly coat the entire surface of a substrate with the protective layer without any variation in the thickness of the layer (thickness variation). It is therefore not possible to completely eliminate the thickness variation caused by the influence of non-uniform drying, non-uniform application, the surface condition of the substrate, or the like. The thickness variation of the protective layer appears as deviations of peaks and valleys in the wavelength in the visible light reflection spectrum of the infrared reflective film, and causes the generation of an iridescent pattern.
When the thickness of the protective layer is increased to a thickness as thick as, for example, several microns, the interval between peaks and valleys decreases in the visible light reflection spectrum of the infrared reflective film, and even if there is some variation in the thickness of the protective layer, it is difficult to distinctively recognize the reflection color at a specific wavelength with the human eyes, and it is therefore almost not possible to perceive an iridescent pattern. Accordingly, the problem of appearance is unlikely to occur. However, the acrylic UV hardcoat agent used for the protective layer contains, in its molecular backbone, a large number of C═O groups, C—O groups and aromatic groups. For this reason, the acrylic UV hardcoat agent easily absorbs far-infrared rays having a wavelength of 5 to 25 μm, and the heat insulation property of the infrared reflective film tends to decrease.
Accordingly, in order to cause the infrared reflective film to have a sufficient heat insulation property, the thickness of the protective layer can be reduced to 1 μm or less so as to suppress the absorption of far-infrared rays having a wavelength of 5 to 25 μm as much as possible. However, as explained in connection with Patent Document 2 above, when the thickness of the protective layer is reduced to several hundred nanometers, which overlaps the wavelength range of visible light, the interval between peaks and valleys increases in the visible light reflection spectrum of the infrared reflective film, and the reflection color at a specific wavelength can be recognized with the human eyes. Accordingly, even if there is a slight variation in the thickness of the protective layer, it is recognized as the iridescent phenomenon. In addition, a change in the reflection color due to a change in the optical path length when viewed from a different angle is also readily perceived, which may present a problem of appearance when it is used by being attached to a window or the like.
Furthermore, as explained in connection with Patent Document 3 above, when the thickness of the protective layer is changed to 150 nm or less, which is smaller than the wavelength range of visible light, the interval between peaks and valleys further increases in the visible light reflection spectrum of the infrared reflective film, and a uniform color is observed as an interference reflection color, and thus the problem of appearance is unlikely to occur. However, the scratch resistance tends to decrease, and thus scratches are likely to be generated on the film surface at the time of the application of the film or while the film is used for a long period of time, which still may present problems caused by the scratches such as poor appearance and corrosion.
As can be seen from the foregoing, it has been difficult to provide a transparent heat-shielding/heat-insulating member that archives both an excellent heat shielding performance in summer and an excellent heat insulation performance in winter, and that has excellent scratch resistance and an excellent appearance that suppresses a reflection color change caused by the iridescent phenomenon and the viewing angle.
The present invention has been made to solve the problem described above, and provides a transparent heat-shielding/heat-insulating member having excellent scratch resistance and an excellent appearance by forming the protective layer of the infrared reflective layer as a stack of layers having specific refractive indices and thicknesses.